| dc.description.abstract | This thesis deals with the studies of magnetization and electrical transport properties of iron nanoparticles embedded in the carbon matrix. The synthesis and characteristics of the nanoparticle systems studied, are also presented.
Carbon-iron (C-Fe) based systems are of growing interest due to their improved magnetic properties as well as in their potential application as sensors, catalysts, and in various other applications. In particular, nanocomposites of iron carbide, such as the cementite phase Fe3C, are further suited to diverse technological exploitations due to their enhanced mechanical properties and importance in ferrous metallurgy. The recent interest in magnetic nanostructures lies in the emergence of novel magnetic and transport properties with the reduction of size. As the dimension approaches the nanometer length scale, interesting size-dependent properties like enhanced coercivity, enhanced magnetic moment, super paramagnetism etc. are seen. Thermal assisted chemical vapour deposition (CVD) is used to decompose and chemically react the introduced precursors, maleic anhydride and ferrocene. This method provides relative size control over the individual particles by varying C/Fe concentration in precursors and the pyrolysis temperature during the co-deposition process. Ferrocene has been used actively for the production of nanoparticle composites and in the production of nanostructured carbon. The temperature of preparation, reaction rate, and the time duration of annealing directly effects the nanoparticle compositions. The catalytic effect of transitional elements are well documented in literature. This thesis is an effort to understand the growth of ferromagnetic nanocrystallites in carbon matrix, which undergo partial graphitization due to the catalytic effect of transitional elements. The effect of transitional metal on the degree of graphitization of the carbon matrix, morphology of the nanoparticle and the carbon matrix are studied. The phase of the ferromagnetic iron nanoparticles and the structural investigation forms part of the study. Here X-Ray diffraction (XRD) is employed to study the presence of different phases of iron in the partially graphitized carbon matrix. The matrix morphology and the particle size distribution were studied using Transmission Electron Microscopy (TEM) and High-Resolution TEM (HRTEM). The ferromagnetic states of the iron nanoparticles are investigated using Mossbauer spectroscopy. The results from these studies, are used to correlated the macroscopic properties to the microscopic studies. The enhanced magnetization, coercivity and the temperature dependence of the magnetization value is understood within the frame work of ferromagnetic Bloch law and surrounding carbon spins. The logarithmic temperature dependence of conductivity of the nanoparticle composites is analyzed in the framework of interference models as well as the many-body Kondo interaction effect.
This thesis contains seven chapters:
In chapter 1, a brief introduction to mesoscopic physics and the size-dependent phenomenon are given. Special attention is paid to magnetic nanoparticle and its composites, and the various finite-size effects exhibited by them are discussed in detail. The relevance of carbon matrix and its importance on the growth of iron nanoparticles with high thermal stability is also discussed. The ballistic and diffusive transport phenomena observed in low-dimensional systems are briefly discussed. The interplay of localization and various interaction effects at nanoscale are examined. In disordered metals the low temperature conductivity is dominated by the interference effects. A brief discussion is made on the conductivity in disorder systems, with the presence of magnetic impurities and how the classic many-body Kondo problem, is effected by various interactions.
Chapter 2, mainly deals with the experimental techniques employed in the thesis. The thermal-assisted chemical vapour deposition setup used to decompose and chemically react the introduced organometallic precursors, for the preparation of C:Fe composites are discussed and its advantage over other preparation methods are emphasized. The method is optimized to provide relative size control over the nanoparticles composites and the phase compositions by varying C/Fe concentration in precursors and the pyrolysis temperature, during the co-deposition process. The various structural characterization tools used in the present study are summed up concisely in this chapter. The SQUID magnetometer system; its working principle and the various protocol used for the low temperature magnetization measurements are elaborated. Further, details regarding superconducting magnetic cryostat, utilized for the low temperature conductivity and magneto resistance measurements, are discussed. Films of C:Fe composites are grown on substrates to study the effect of disorder and sample size on the conductivity behaviour of the composites at low temperature.
Chapter 3, presents the outcome of the structural studies undertaken on the C:Fe composites using XRD, TEM, and HRTEM. X-ray diffraction measurements performed on the powder composites reveal that, in addition to the presence of sharp diffraction peak from nanographite, peaks corresponding to the different phases of Fe are also seen. The effect of preparation temperature on the matrix morphology is revealed from the estimation of degree of graphitization. Iron carbide is the predominant phase in all the prepared composites. For low concentration of iron, iron carbide alone is present but as the percentage of iron in the samples increased other phases of iron are also seen. The microscopic studies on the prepared compositions revealed the presence of nanosized iron particles well embedded in the partially graphitized matrix. Here again, with the increase in iron percentage, agglomeration of ferromagnetic nanoparticles are seen. The kinetics of the particle growth and the filamentous nature of the carbon matrix are also discussed.
Mossbauer investigation on C:Fe composites are presented in chapter 4. The measurements revealed the iron atom occupation in the crystal lattice. In the lower Fe concentration samples, the room temperature Mossbauer spectrum revealed the presence of sextet from Fe3C (cementite) phase. As the percentage of iron increased, sextet from α-Fe, Fe3O4 are also seen in some of the prepared compositions. Effect of carbon atoms on the structure and magnetic properties of the nanoparticle species are obvious from the isomer shift measurements.
Chapter 5 comprises of the various magnetic properties and interactions present in small particle system such as magnetic anisotropy, coercivity, enhanced magnetization, inter-and intra-particle interactions etc. Magnetization measurements carried out in SQUID magnetometer on the C:Fe composites and carbon flakes (prepared from organic precursor, maleic anhydride alone) are presented. The enhanced magnetic properties of the nanoparticle assembly is discussed in detail. The hysteresis loops trace, with a finite coercivity at room temperature, indicates the ferromagnetic nature of the samples. At room temperature the magnetization value saturates at high magnetic field, indicating negligible effect from super paramagnetic particles on the hysteresis loop. The squareness ratio, saturation magnetization, coercivity and remanence magnetization values are analyzed in detail. The temperature dependence of magnetization shows a combination of Bloch law and Curie-Weiss behaviour, consistent with the picture of ferromagnetic clusters embedded in a carbon matrix. The Bloch’s constant is found to be larger by an order of magnitude compared to the bulk value, implying stronger dependence of magnetization with temperature. Effort to understand the enhanced magnetic moment in the light of magnetism in carbon was taken up. The proximity effect of ferromagnetic metal on the carbon and the hydrogen bonding with the dangling bonds, both studied in detail in literature, in connection with the induced magnetic moments in carbon, are invoked.
In chapter 6, the different conductivity regimes are identified, to study the conduction mechanisms in composites and films. For the transport measurements pelletized samples are used for the resistivity and magneto resistance measurements. The conductivity data are analyzed based on the interplay of localization and Kondo effect in the ferromagnetic disordered system. In order to understand the effect of disorder and thickness on the Kondo problem, transport measurements are carried on thin films of C:Fe composites grown on quartz and alumina substrate. Disorder induced metal-insulator transition is observed in the prepared samples. The zero-field conductivity and magneto resistance data is fitted to variable range hopping (VRH) in strong localization regime.
Chapter 7 summarizes the thesis and presents some perspectives for the future. | en |
